1. Field Of The Invention
The present invention relates generally to stable ascorbic acid compositions that are homogeneous solutions suitable for topical use. More specifically, the present invention relates to homogeneous cosmetic preparations incorporating stabilized ascorbic acid that can be topically applied to human skin to impart beneficial appearance effects thereto.
2. Description Of The Prior Art
It is well known that ascorbic acid, synonymously referred to herein as Vitamin C, is essential to the maintenance of a healthy and attractive skin appearance in humans. Vitamin C helps to stimulate and regulate the production of collagen in human skin tissue thus retarding the formation of wrinkles and helping to maintain a healthier and younger looking appearance. Vitamin C also helps to minimize lipid oxidation and cellular damage resulting from prolonged exposure to the sun's ultraviolet rays. By doing so, it counteracts premature aging of the skin. It is generally believed that ascorbic acid inhibits the formation of melanin that can lead to skin discoloration during the aging process, and inhibits the release of histamine from cellular membranes that can lead to allergenic reactions, particularly among individuals with sensitive skin.
Because of these many beneficial effects, it has long been desired to deliver effective concentrations of ascorbic acid percutaneously and directly to the underlying tissue matrix of the skin, i.e. the dermal layer, via a topically applied and cosmetically elegant carrier or base.
Ascorbic acid is appreciably soluble in water. However, it oxidizes rapidly in aqueous solutions. Therefore, it cannot be stabilized in such a medium at the sufficiently high concentrations required to achieve the desired appearance enhancement effect on the skin. On the other hand, solubility of ascorbic acid in non-aqueous media is quite limited so that a large amount of a solvent, such as ethanol, is required to dissolve limited amounts of ascorbic acid. Thus, the limited solubility of ascorbic acid has, heretofore, prevented anhydrous and low water systems from achieving ascorbic acid concentration and efficacy levels required for producing the above described appearance enhancement effect. For this reason, ascorbic acid has been heretofore used in cosmetic formulations only in relatively low concentrations. As a result, prior attempts to develop and to market acceptable cosmetic formulations containing efficacious concentrations of stabilized ascorbic acid have generally met with limited success.
Ascorbic acid is unstable in the presence of oxygen and is rapidly oxidized to dehydroascorbic acid. In body cells, glutathione can reverse the ascorbic acid dehydroascorbic acid conversion so that a pool of ascorbate distributed throughout the body can be maintained.
Aside from preventing scurvy, ascorbic acid is essential to many body or biological functions. The most notable function is the synthesis of collagen, the major fibrillar component of dermal connective tissue, comprising approximately 70 to 80% of the dry weight of the dermis. The essential role of ascorbic acid in the formation and maintenance of collagen as well as in other biological functions and processes has been summarized in U.S. Pat. No. 4,983,382 to Wilmott, et al, which is owned by the assignee of the present invention. The contents of U.S. Pat. No. 4,983,382 are incorporated herein by reference.
U.S. Pat. No. 4,983,382 suggests that replenishing dermal tissues with ascorbic acid percutaneously delivered through the stratum corneum can impart beneficial appearance effects to the skin, including improved tone and luster, a decrease in fine lines and wrinkles, and improved elasticity. However, the compositions disclosed in that patent have ethanol at a concentration of at least forty percent. These compositions do not exhibit sufficient stability to maintain high levels of ascorbic acid concentrations for sufficiently long periods of time. Therefore, they do not generally possess the performance level necessary for the above mentioned skin appearance enhancement effect in cosmetic formulations. To overcome this shortcoming, it is recommended in U.S. Pat. No. 4,983,382, described above, and also in U.S. Pat. No. 5,140,043, to Darr, to store the cosmetic formulation samples in bottles that are impervious to ultraviolet light and to preferably provide with a headspace containing an inert gas.